Direct visualization of stacking-selective self-intercalation in epitaxial Nb1+xSe2 films

Two-dimensional (2D) van der Waals (vdW) materials offer rich tuning opportunities generated by different stacking configurations or by introducing intercalants into the vdW gaps. Current knowledge of the interplay between stacking polytypes and intercalation often relies on macroscopically averaged probes, which fail to pinpoint the exact atomic position and chemical state of the intercalants in real space. Here, by using atomic-resolution electron energy-loss spectroscopy in a scanning transmission electron microscope, we visualize a stacking-selective self-intercalation phenomenon in thin films of the transition-metal dichalcogenide (TMDC) Nb1+xSe2. We observe robust contrasts between 180°-stacked layers with large amounts of Nb intercalants inside their vdW gaps and 0°-stacked layers with little detectable intercalants inside their vdW gaps, coexisting on the atomic scale. First-principles calculations suggest that the films lie at the boundary of a phase transition from 0° to 180° stacking when the intercalant concentration x exceeds ~0.25, which we could attain in our films due to specific kinetic pathways. Our results offer not only renewed mechanistic insights into stacking and intercalation, but also open up prospects for engineering the functionality of TMDCs via stacking-selective self-intercalation.

Supplementary Table 1 Growth parameters of Nb1+xSe2 films.a Deposition of the first layer; b deposition of the subsequent layers; c post-growth annealing.The spot size of the laser pulse on the target is estimated to be 1 mm 2 .Supplementary Fig. 4 shows low-energy electron diffraction (LEED) images of two Nb1+xSe2 films, NbSe183 and NbSe208, acquired at two electron energies each.Since the films are 6-10 layers thick, we only observed diffraction spots from the hexagonal NbSe2 lattice, and none from the Al2O3 substrate.

Sample
As seen in Supplementary Fig. 4(a), there is a series of six primary diffraction spots (solid circles), as well as another series of secondary diffraction spots (broken circles), rotated by 30.The secondary diffraction spots correspond to domains that are rotated by 30, which were also seen in the STEM crosssectional images (Figs.1(e) and 1(f) of the main text, as well as Supplementary Fig. 3 and 5).Such domain rotation is common in layered vdW compounds grown via molecular beam epitaxy where p is the mixing factor.p = 100 represents a pure Lorentzian, whereas p = 0 represents a pure Gaussian.
FWHM stands for the full width at half maximum.
measurements.(a) STM topographic image of a Nb1+xSe2 film (sample NbSe180) acquired at room temperature.Set point: 1 V, 20 pA.(b) Row-averaged height profile across the red line in (a).(c) Atomically resolved topographic image (reproduced from Fig. 1(d) of the main text).Each bright spot corresponds to the topmost Se atom in a Se-Nb-Se layer.Set point: −125 mV, 50 pA.(d) Gaussian convolution of (c) with a width of 15 pixels, revealing the background inhomogeneity in (c).(e) Atomically resolved topographic image of a second film (sample NbSe194).Set point: 250 mV, 100 pA.Supplementary Figs.2(a) and 2(b) show a topographic image and a line cut across multiple step edges.Higher resolution images over smaller fields of view reveal a hexagonal atomic lattice of Se atoms [Supplementary Figs.2(c) and 2(e)].STM images such as these ones can reveal the presence of various defects, including vacancies, adatoms, and intercalants.Vacancies would appear as missing bright spots in the hexagonal lattice, whereas adatoms would appear as distinct, large bright spots on top of the hexagonal lattice.What we observe are not vacancies or adatoms, but an inhomogeneous background to the hexagonal lattice, with brighter and darker patches.To better visualize this background inhomogeneity, a Gaussian convolution was applied in Supplementary Fig.2(d) to remove the atomic corrugations.This disorder likely arises from the subsurface Nb intercalants1 and is consistent with STEM images showing varying intensities at Nb intercalant sites.For example, the area enclosed by the orange dashed line has a brighter intensity and may correspond to a region with more Nb intercalants, whereas the area enclosed by the green dashed line has a darker intensity and may correspond to a region with fewer Nb intercalants.We can also deduce that the intercalants do not exhibit in-plane ordering, and are inhomogeneous on the length scale of nanometers.Supplementary Fig. 4 LEED results.LEED images of samples (a), (b) NbSe183 and (c), (d) NbSe208.The solid circles in (a) mark the dominant diffraction spots from the hexagonal NbSe2 lattice, whereas the broken circle marks secondary diffraction spots from domains rotated by 30.The electron energies are shown in the upper right corner of each panel.(e), (f) ARPES constant energy cuts of sample NbSe208 at the Fermi energy EF and -0.725 eV below EF.In (e), the Brillouin zone and its 30 rotations are overlaid.

Table 2 .
Se2) is around 0.6-0.7 eV, which is close to the energy difference of components A and B, 0.79 eV.Note that there are two values for Se2 in Supplementary Fig.10(c), because there are two crystallographically inequivalent Se sites with fourfold coordination.Furthermore, the percentages of Se1 and Se2 atoms are 25% and 75%, respectively, which would yield x = 0.75/3 = 0.25.XPS fitting parameters.GL(p) stands for a Gaussian-Lorentzian mixture function,